In the PPD case, the inlet bubble gas pressure [P.sub.g0] is estimated from the

solution pressure reading at [P.sub.1].

3 where [P.sub.s] is the nucleated solution pressure entering the die slit, [P.sub.exit] is the system pressure at the die exit, and L is the die slit length.

The first case accounts for the fact that gas diffuses into the cells as the solution pressure decreases such that cell growth during the die flow is driven by both the gas pressure in the cells and the diffusion of gas into the cells.

In general, the solution pressure links shaping to cell growth and cell growth to nucleation.

The design parameters chosen to satisfy these requirements are the nucleated solution pressure during shaping, [P.sub.s], a mechanical constraint, and the die temperature, [T.sub.die].

As the

solution pressure drops further, the system will either nucleate additional microcells and expand the existing cells by gas diffusion or only expand the existing cells.